The present invention relates to a process for preparation of pure methacrylic acid, at least comprising the process steps: a) gas phase oxidation of a C4 compound to obtain a methacrylic acid-comprising gas phase, b) condensation of the methacrylic acid-comprising gas phase to obtain an aqueous met
The present invention relates to a process for preparation of pure methacrylic acid, at least comprising the process steps: a) gas phase oxidation of a C4 compound to obtain a methacrylic acid-comprising gas phase, b) condensation of the methacrylic acid-comprising gas phase to obtain an aqueous methacrylic acid solution, c) separation of at least a part of the methacrylic acid from the aqueous methacrylic acid solution to obtain at least one crude methacrylic acid-comprising product; d) separation of at least a part of the methacrylic acid from the at least one crude methacrylic acid-comprising product by means of a thermal separation process to obtain a pure methacrylic acid.
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1. A process for preparation of pure methacrylic acid, comprising: a) carrying out gas phase oxidation of a C4 compound to obtain a methacrylic acid-comprising gas phase,b) condensing the methacrylic acid-comprising gas phase to obtain an aqueous methacrylic acid solution,c) separating at least a pa
1. A process for preparation of pure methacrylic acid, comprising: a) carrying out gas phase oxidation of a C4 compound to obtain a methacrylic acid-comprising gas phase,b) condensing the methacrylic acid-comprising gas phase to obtain an aqueous methacrylic acid solution,c) separating at least a part of the methacrylic acid from the aqueous methacrylic acid solution to obtain at least one crude methacrylic acid-comprising product; andd) separating at least a part of the methacrylic acid from the at least one crude methacrylic acid-comprising product by a thermal separation process to obtain a pure methacrylic acid;wherein said thermal separation process is a distillation or rectification;wherein in d), the methacrylic acid is separated by rectification from at least a part of the at least one crude methacrylic acid-comprising product, wherein the pure methacrylic acid is removed in a side outlet from a column used for the rectification; andwherein the amount of pure methacrylic acid removed in a defined time interval is 40 to 80% of the amount of the amount of crude methacrylic acid-comprising product introduced into the rectification column in the same time interval. 2. The process according to claim 1, further comprising in c): c1α) extracting the methacrylic acid from the aqueous methacrylic acid solution by an organic extraction agent to obtain an aqueous phase and an organic phase,c1β) separating at least a part of the organic extraction agent from the organic phase by at least one thermal separation process to obtain at least one methacrylic acid-comprising bottom product as the crude methacrylic acid-comprising product. 3. The process according to claim 2, wherein the organic extraction agent used in c1α) has a boiling point of less than 161° C., measured at atmospheric pressure. 4. The process according to claim 1, further comprising in c): c2α) crystallizing at least a part of the methacrylic acid from the aqueous methacrylic acid solution,c2β) optionally, washing the crystallised methacrylic acid, andc2g) melting at least a part of the crystallised methacrylic acid to obtain at least one crude methacrylic acid-comprising product. 5. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product introduced into d) is obtained by distillation or crystallisation of a composition which comprises at most 95 wt. %, methacrylic acid. 6. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product introduced into d) is obtained by distillation or crystallisation of a composition which comprises at least 5 wt. % of a C4-C8 hydrocarbon. 7. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product introduced into d) has an APHA number according to DIN ISO 6271 of at least 100. 8. The process according to claim 1, wherein the rectification in d) is carried out at a bottom pressure in a range from 1 to 100 mbar. 9. The process according to claim 1, wherein the rectification in d) is carried out at a bottom temperature in a range from 40 to 200° C. 10. The process according to claim 1, wherein the rectification in d) is carried out at a bottom temperature of less than 90° C. 11. The process according to claim 1, wherein the rectification column has more than 0 and up to 10 theoretical plates per meter. 12. The process according to claim 1, wherein the pure methacrylic acid obtained in d) is removed at a height in a range between a lower fourth and an upper fourth of the rectification column. 13. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product introduced into d) is subjected to a flow resistance at or after entering into the rectification column. 14. The process according to claim 13, wherein the at least one crude methacrylic acid-comprising product entering the column impinges on the flow resistance at an angle in the range from 60° to 120°, based on a main flow direction at the flow resistance of the crude methacrylic acid-comprising product entering the column. 15. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product obtained in c) is introduced into the bottom of the rectification column. 16. The process according to claim 1, wherein the at least one crude methacrylic acid-comprising product obtained in c) is introduced into the rectification column from 1 to 5 theoretical floors below the side outlet. 17. The process according to claim 1, wherein the thermal separation in d) is carried out in the presence of a decolourisation agent. 18. The process according to claim 17, wherein the decolourisation agent is an aminoguanidinium salt. 19. The process according to claim 18, wherein the aminoguanidinium salt is aminoguanidinium bicarbonate. 20. The process according to claim 17, wherein the decolourisation agent is added A) to the at least one crude methacrylic acid-comprising product obtained in c) before the thermal separation in d), orB) during the thermal separation in d). 21. The process according to claim 1, wherein in d), less than 5 wt. %, impurities different from methacrylic acid are precipitated from the at least one crude methacrylic acid-comprising product obtained in c). 22. The process according to claim 1, wherein at least d) is carried out continuously. 23. The process according to claim 1, wherein the process is carried out in a device, wherein the device comprises at least the following components in fluid-conducting communication with each other: a1) a gas phase oxidation unit;b1) an absorption unit;c1) a separation unit; andd1) a purification unit;wherein the purification unit comprises at least one distillation column and the at least one distillation column comprises at least one side outlet for pure methacrylic acid. 24. A process for preparation of methacrylic acid esters, comprising A) preparing pure methacrylic acid by a process according to claim 1, andB) esterifying the pure methacrylic acid. 25. A process for preparation of a polymethacrylate, comprising i) preparing pure methacrylic acid by a process according to claim 1, andii) carrying out radical polymerisation of the pure methacrylic acid, optionally in the presence of monomers which are co-polymerisable with methacrylic acid. 26. A process for preparation of a polymethacrylic acid ester, comprising i) preparing a methacrylic acid ester by a process according to claim 24, andii) carrying out radical polymerisation of the methacrylic acid ester, optionally in the presence of monomers which are co-polymerisable with methacrylic acid esters. 27. The process of claim 1, wherein said pure methacrylic acid is a methacrylic acid which comprises less than 1 wt. % impurities, based on the total weight of methacrylic acid and impurities. 28. The process of claim 1, wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.8 wt. % impurities, based on the total weight of methacrylic acid and impurities. 29. The process of claim 1, wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.5 wt. % impurities, based on the total weight of methacrylic acid and impurities. 30. The process of claim 1, wherein said pure methacrylic acid is a methacrylic acid which comprises less than 0.3 wt. % impurities, based on the total weight of methacrylic acid and impurities.
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